1. Field of the Invention
The present invention relates generally to a wireless sensor network, and more particularly to a method for transmitting/receiving required data between nodes in a wireless sensor network in which a Time Division. Multiple Access (TDMA) scheme is employed.
2. Description of the Related Art
A wireless sensor network collects remote information differently from existing communication networks. Such a wireless sensor network generally includes sensor nodes for processing and transmitting information collected through sensors, and a sink node for collecting and processing information transmitted from the sensor nodes.
Since the wireless sensor network includes many sensor nodes, the design of the sensor nodes must be simple. Also, since the sensor nodes are located in areas that are difficult to access, the sensor nodes should be designed to require low power to minimize the need for replacement. In addition, the sensor nodes should be designed to have mobility so that the location of installation is easily changeable. Furthermore, the wireless sensor network should be designed such that any damaged node does not have an influence on the maintenance of the network.
FIG. 1 illustrates a conventional wireless sensor network. In FIG. 1, each node located within a sensor network forms a network with adjacent nodes. In order to facilitate the formation of such a network, the configuration of the nodes may also be simply designed. When a network as described above is formed, an upper-lower relation is formed between nodes. Although FIG. 1 illustrates a mesh network, the relation between nodes can be formed even within the mesh network since a tree structure also exists in the mesh network.
The standard a short-range wireless network is defined in IEEE 802.15 Working Group. Particularly, the IEEE 802.15.4 standard defined by the IEEE 802.15 Working Group is thought of as a prominent core technology for application to a sensor network since it relates to the implementation of short-range wireless network with low power.
Hereinafter, a Method for transmitting/receiving data in a wireless sensor network based on the IEEE 802.15.4 standard protocol will be briefly described.
FIG. 2 illustrates data transmission/reception between nodes using time slots in a conventional wireless sensor network based on the TDMA scheme. For conciseness of the description of FIG. 2, the following describes one upper node and five lower nodes being located within a corresponding wireless sensor network, and communicating with each other.
In FIG. 2, one upper node 201 forms a connection with a plurality of lower nodes 203 to 211, which are to communicate with the upper node 201. The plurality of lower nodes, defined as first to filth lower nodes, are usually in a sleep mode so as to save energy: When a predetermined time is reached, a corresponding lower node wakes up from the sleep mode, and enters an active mode to communicate with the upper node 201. For example, when it is assumed that the upper node 201 is in communication with the first lower node 203, only the first lower node 203 has entered the active mode, and the other lower nodes 205 to 211 remain in the sleep mode. That is, the respective lower nodes 203 to 211 communicate with the upper node 201 only within each corresponding time slot, which is allocated to each of the lower nodes 203 to 211.
However, according to the allocation method, each lower node must wake up from the sleep mode and enter the active mode to communicate with the upper mode, unconditionally, within a time slot allocated to the lower node. Consequently, each lower node enters the active mode regardless of whether there is data to be transmitted, which wastes energy in a network environment in which the ratio of active mode to sleep mode is very low and an event occurs at infrequent intervals. For example, in FIG. 2, although the third lower node has no data to be reported to the upper node, the third lower node enters the active mode and communicates with the upper node when the time to enter the active mode is reached.
Accordingly, when the frequency of occurrence is low, each lower node enters the active mode to communicate with the upper node although the lower node has no data to be transmitted to the upper node, resulting in wasted energy consumption.